1. Field of the Invention
The present invention relates to a free space optical communication link, and in particular, the invention relates to techniques to spread error sources over time intervals and provide redundant channels to reduce the effects of fading
2. Description of Related Art
Known optical communication systems rely on optical fibers between transmitter and receiver. However, to establish a system network require obtaining right of ways and installation of fiber, a time consuming and expensive process.
Free space optical communication systems are fundamentally different than fiber optic systems. Distances are more limited. The media is air and subject to atmospheric disturbances such as fog rain and resulting fades.
Koh and Davidson (“Interleaved Concatenated Coding For The Turbulent Atmospheric Direct Detection Optical Communication Channel”, IEEE Transactions On Communications. Vol. 37. No. 6, June 1989, pages 648-651) discuss how the direct detection atmospheric optical communication channel is characterized by strong fading light intensity caused by random variations in the index of refraction light variations as it propagates through the channel.
In addition, the Jet Propulsion Laboratory of the California Institute of Technology published in November of 1998 a Technical Support Package on Multiple Beam Transmission For Optical Communication in November 1998 as NASA Tech Brief, Vol. 22, No. 11 from a JPL New Technology Report NPO-20384. This NASA Tech Brief describes how superposition of mutually incoherent beams would reduce deleterious effects of atmospheric turbulence.